Apolipoprotein (apo) B-containing lipoproteins include the chylomicrons, VLDL and IDL, and LDL and Lp(a). ApoB-100 is associated with VLDL IDL LDL and Lp(a); apoB-48 is associated with chylomicrons and their remnants. ApoB-100 and apoB-48 are the products of the same gene. The biogenesis of apoB-48 is unique. The apoB-48 mRNA is the product of RNA editing, whereby the first nucleotide (C) for the codon CAA, encoding Gln-2153, in apoB-100 mRNA is converted to U producing an in-frame stop codon. As result of this change, translation of apoB-48 mRNA produces a 2152 aa apoB-48 protein instead of the 4526 aa apoB-100. ApoB mRNA editing occurs in the small intestine only in humans, but in both the liver and small intestine in rats and mice. ApoB-100 and apoB-48 have vastly different properties. All apoB-100 containing lipoproteins are highly atherogenic, whereas apoB-48 is required mainly for fat absorption. The long- term objectives of this proposal are to determine the subunit composition of the apoB mRNA editing complex and to create mouse models that simulate humans in their tissue distribution of apoB mRNA editing. The specific aims are (i) to identify proteins that interact with apobec-1, the newly cloned catalytic subunit of the multicomponent editing enzyme complex. Two techniques will be used, a) the yeast two hybrid system and b) characterization of the editing enzyme complex in apobec-1-expressing cell line and, if necessary, in transgenic mice. 11) To create mice with an inactivated apobec-1 gene. Editing will be abolished in all tissues by gene targeting and homologous recombination. The apobec-1 null animals will be a useful model to study the relative roles of apoB-100 and apoB-48 in lipoprotein metabolism: other unexpected phenotypic effects may also provide information on other mRNAs that may be edited by this mechanism. iii) To create mice with apoB mRNA editing in the small intestine only by a liver-specific knockout of the apobec-1 gene by a variety of techniques, and by restoration of apobec-1 function in the intestine of the apobec-1 mice using a transgenic technique. The "humanized" mice with intestine-specific editing will be a useful model for lipoprotein and atherogenesis experiments. The project may provide important information related to lipoprotein metabolism and atherosclerosis and a novel pathway involved in the posttranscriptional regulation of gene expression.